Hi Alfred
I don't know the answer to this one but I can tell you that you are not alone as my 1997 323 LXi does exactly the same thing even after I had it serviced. My central locking has also gone now, first the rear doors then the passenger front door and now the driver's side door.
I hope there is someone with a bit if technical knowledge out there who can come up with a fix for this.
John
December 2007